Fibers have long been used to reinforce mechanical properties of composite plastic structures, but such structures often fail in use under stress conditions much lower than are predicted from the strength of its individual components. For example, structures having a combination of a plastic matrix, with only a modest level of tensile strength, in which are embedded glass fibers, with an enormously high level of this property, in theory and preliminary testing should have a very high level of performance but in use fail unexpectedly. Recent investigations have revealed that in such failures, fractures, or irreversible deformations, the pulling out of the fibers from their sockets in the plastic matrix occurs. The result is little if any reinforcement of the structures by the fiber. High energy to pull out leads to strong, tough materials.
In current approaches for increasing pullout energy, fibers are coated with compositions which provide chemical coupling between the fiber and the matrix. For example, organo-silicon compounds are used to bond organic matrix to a silica-based glass fiber. In spite of this, failures occur; the bonds so formed are inherently weaker than other components of the structure.
Another approach in efforts to increase the energy to pull out is by increasing the contact surface between the fiber and the matrix. Fibers of complex cross sections (e.g., petaloid) have been used, but without a significant decrease in the aforementioned type of failures.
Concrete, a rigid structure with a very high compressive strength but a very low tensile strength, employs steel reinforcement rods which have surface deformations to more tightly engage the rigid concrete matrix. These deformations are typically small (approximately 5-8% of the rod diameter), widely spaced, narrow ridges which extend, circumferentially, part way around the rod. The concrete, which has a low thermal expansion (and contraction), is formed around these rods at temperatures near use temperatures, so there is little tendency for the matrix to become disengaged from the reinforcement. Reinforcement of concrete, a rigid material with behavior and use characteristics decidedly different from plastic, provides no clues as to the causes of the failures in fiber-reinforced plastic structures mentioned.